Transcutaneous Electrical Nerve Stimulation (TENS; see also FIG. 1) is a commonly used forms of electroanalgesia (electrical pain relief). Hundreds of clinical reports exist concerning the use of TENS for many types of conditions such as low-back pain, myofascial and arthritic pain, sympathetically mediated pain, bladder incontinence, neurogenic pain, visceral pain and post-surgical pain. TENS is the application of electrical stimulation at the surface of the skin (=transcutaneous), primarily for pain relief. TENS is applied via external surface electrodes with some sort of electrical waveform characterized by frequency, pulse duration and amplitude. The technique of applying electricity for relieving pain well established but gained substantial scientific interest after 1965 after a scientific base was established on the mechanism of pain reduction. TENS is drug-free, noninvasive, and non-addictive. It has hardly any contraindications.
A disadvantage of known TENS devices and methods is that these do not deal with fluctuations in skin conductivity due to for instance movement, moisture build-up or changing temperatures. In known systems this may have the following consequences
In case of very poor connection between skin and electrode, stimulation can cause voltage to rise significantly=>potentially undesirable, painful experience.
an inconvenient or cumbersome fail-safe mechanism in case of frequent changes of resistance levels. If the device enters the fail-safe state, the user needs to restart the device frequently. This is not user-friendly.
an earlier selected stimulation point might no longer be the right point